In patients with pulmonary arterial hypertension, a known or unknown disease process results in vasoconstriction and proliferation of the cells making up the wall of the small pulmonary arteries. This leads to increased resistance to blood flow and increased steady state pressure in the pulmonary artery. Over time, the increased pressure in the pulmonary artery, as well as other disease processes, cause the pulmonary artery to lose its elasticity, resulting in a decrease in vascular compliance. Vascular compliance is a measure of the elastic properties of a vessel and is defined as the change in volume in a vessel in response to a change in pressure (ΔV/ΔP). A compliant vessel is able to accommodate a relatively large volume change for a given change in pressure. With each stroke of the heart, a volume of blood (stroke volume) is pumped from the right ventricle into the pulmonary artery. When the compliance is low, as occurs in pulmonary arterial hypertension, the right ventricle must produce a high pressure in order to pump each stroke volume into the pulmonary artery because the vessel is unable to stretch to accommodate the incoming blood. This results in a high pulse pressure, which is the arithmetic difference between the systolic and diastolic pressures.
The abnormally high stiffness of the arterial wall also affects the pulse wave velocity (PWV) so that reflected waves may contribute significantly to pulse pressure. PWV may be estimated by the Moens-Korteweg equation: PWV=(E h/2Rρ)1/2. An important component of this equation is E or Young's modulus, a measure of stiffness of the arterial wall. In pulmonary arterial hypertension, Young's modulus may be greater than normal, resulting in a higher than normal PWV. With each heartbeat and ejection of blood, a temporary and localized increase in hydraulic pressure is created in the pulmonary artery. This pressure impulse propagates away from the heart as an acoustic wave. When the wave encounters an impedance discontinuity, such as an abrupt change in diameter or a branch, a reflection occurs. These reflections travel retrograde towards the heart. In a person with normal vascular stiffness and PWV, the major reflected wave reaches the heart after the ejection of blood. But with aging and the development of systemic hypertension, pulmonary arterial hypertension, arteriosclerosis, as well as other conditions, vascular stiffness and PWV may be increased, causing the major reflected wave to arrive at the heart earlier. In many patients with pulmonary arterial hypertension, the major wave can arrive during ejection, significantly contributing to the pulmonary artery pulse pressure and cardiac work.
In 1983, Sunagawa employed the concept of elastance as a change in pressure over change in volume (elastance is ΔP/ΔV, the reciprocal of compliance). (Sunagawa at al., Am J Physiol Heart CircPhysiol 245: H773-H780, 1983.) In normal, healthy individuals, the elastance of the pulmonary artery and the right ventricle are matched or coupled. Much like impedance matching in an electrical circuit, coupling represents a state of optimum stroke work and energetic efficiency. (Borlaug et al., Heart Fail Clin 4: 23-36, 2008.) The optimum value of the ratio of elastance of the right ventricle to the elastance of the pulmonary artery is between 1 and 2. (Naeitje et al., Eur Heart Journal Supplements (2007) 9 (supplement H), H5-H9.) In advanced stages of pulmonary arterial hypertension, this ratio is decreased, a condition termed afterload mismatch. (Borlaug, Ventricular-Vascular Interaction in Heart Failure, Heart Failure Clin 4 (2008) 23-36.) This decoupling indicates that additional energy is needed in order to maintain flow, thus imposing additional load on the right ventricle.
Thus, as appreciated by those of ordinary skill in the art, the relatively low compliance (or high stiffness) of the pulmonary artery in patients with pulmonary arterial hypertension leads to increased pulse pressure. It also leads to higher PWV, which causes reflected waves to contribute to afterload, further increasing pulse pressure. Furthermore, elastance decoupling leads to a state of energetic inefficiency and increased workload for the heart. These components combine to contribute to pulsatile load and increase the workload on the right ventricle.
In pulmonary arterial hypertension, the total load that the right ventricle must overcome to pump blood can be considered the sum of the steady state load (due to restriction of flow caused by small vessel constriction) and the pulsatile load (which is caused by decreased compliance or increased stiffness). While in the past most researchers and physicians focused on addressing the steady state load, many researchers now feel that pulsatile load may be of comparable importance in imposing a load on the heart.
In a normal, healthy individual, the pulmonary circulation operates at a substantially lower pressure than the systemic circulation. The pressure in the right ventricle is usually one-sixth of that in the left ventricle. In comparison to the left ventricle, the right ventricle is less capable of withstanding chronically elevated pressures and workloads. Initially, when exposed to high pressure the right ventricle adapts to the higher load via multiple mechanisms including hypertrophy, but as the pressure continues to rise, the heart loses this ability to compensate, eventually leading to right heart failure, a leading cause of death in pulmonary arterial hypertension.
Drugs are the mainstay of current therapy for pulmonary arterial hypertension. An important function of the pulmonary arterial hypertension-specific drugs is to dilate the small pulmonary arteries. These medications tend to lower the steady state load by increasing the cross-sectional area of the constricted vessels, but do not directly target the elevated pulsatile load caused by lack of compliance.
To summarize, many pulmonary arterial hypertension patients die of right heart failure due to chronically elevated load on the right ventricle. Increased pulsatile load is a significant component of the total load and is caused by a relative lack of compliance in the pulmonary artery. Current therapy is not directed at improving compliance. Thus, there is a need for a solution to lower pulsatile load by increasing compliance of the pulmonary artery.